You can try to dispute the payment in your payment processor (PayPal). However it might not be possible to get the money back as the purchase might have been an unauthorized credit card transaction.
Suggestions if you wish to keep selling bitcoins on high risk payment methods:
Follow our risk mitigation guide when trading on a high risk payment methods.
Enable real name and confirmed phone number requirements on your advertisement options to make sure the person doing the payment is the one he/she claims to be.
Use LocalBitcoins.com Trusted listings where you can advertise only for persons who you trust.
If you provide us a police report we can provide details regarding the fraudulent transaction (IP address, email, bitcoin addresses) etc...